1. Field
This disclosure relates generally to semiconductor devices, and more specifically, to packaging for semiconductor sensor devices.
2. Related Art
Gel material, particularly silicone gels, are used for encapsulation of micro electro-mechanical systems (MEMS), sensors, and actuators during the packaging process. Frequently, the gel must have a very low modulus to minimize thermomechanical stress on the MEMS, sensor, or actuator element. The application environment for the end product may require resistance to mechanical stress. As an example, a tire pressure monitoring system (TPMS) is subjected to high rates of rotation and the stress the rotation induces. The centrifugal stress will cause a very soft gel that is desired for low thermomechanical stress to move, or flow from the initial area of application and cure, possibly reducing the reliability of the TPMS. As a consequence, a gel with a modulus that is higher than desired from the perspective of minimized thermomechanical stress is used to resist movement or flow during centrifugal testing and in the application environment. The higher modulus gel can induce stresses on wire bonds in the package, which can result in wire breaks with temperature cycle testing and in the operating environment. Further, the increased modulus can impact the performance of the MEMS, sensor, or actuator element with which the gel is in contact. Thus, two conflicting criteria are considered when selecting gel for use in encapsulated devices with wire bonds.